Optical lithography is a technology used to print patterns that define integrated circuits (IC) onto semiconductor wafers. Typically, a pattern on a photomask is imaged by a highly accurate camera called a stepper or scanner and transferred onto a semiconductor wafer coated with photoresist. Continued improvements in optical lithography have enabled the printing of ever finer features. This has allowed the IC industry to produce more powerful and cost-effective semiconductor devices.
A conventional binary mask that controls the amplitude of light incident upon a wafer is often inadequate when the IC feature size is small. Under sub-wavelength conditions, additive amplitude effects, optical distortions as well as diffusion and loading effects of photosensitive resist and etch processes may cause printing aberrations. Phase shifting improves the resolution that optical lithography can attain, producing smaller, higher-performance IC features by modulating the projected light at the mask level. Phase-shifting masks may be used, for example, for optical lithography for the generation of IC feature sizes below one micron such as 0.25 micron.
Successors to optical lithography are being developed to further improve the resolution. Extreme-ultraviolet (EUV) lithography is one of the leading successors to optical lithography. It may be viewed as a natural extension, since it uses short wavelength optical radiation to carry out projection imaging (i.e.—a wavelength on the order of about 13 nm). However, EUV lithography technology may be different from preceding technologies in that properties of materials with respect to EUV radiation are different from their properties with respect to other types of radiation such as visible and deep ultraviolet (DUV) ranges. For example, the EUV radiation is strongly absorbed in most materials, including gas. Thus, EUV imaging systems often utilize entirely reflective optical elements rather than refractive elements, such as lenses.
The attenuation and phase for current phase-shift masks are typically obtained by a single layer attenuating film. As a result, the attenuation is a fixed value. Also, the current phase shift masks often undergo multiple patterning steps to darken the peripheral region of the mask. The peripheral region is darkened to prevent light leakage and minimize any mask flare effects. Improvements to the phase shift mask structure that allow for tunable attenuation and peripheral darkening without additional patterning/etching steps may provide many benefits including cost savings from fewer process steps, more latitude in attenuation range, and ability to use the same deposition and etch chemistries for different attenuation.